It is the purpose of this project to analyze and develop new and difficult systems for cell culture. Four related but separate problems are now active in our laboratory. I. For 30 years the usual goal of in vitro cell biology has been to fragment the more complex tissues and organs from well characterized, cloned cell strains from the rat thyroid gland: follicle cells, c-cells, fibroblasts and parathyroid cells. These cell combinations have been tested as grafts in thyroidectomized animals. II. In a collaborative effort we are examining the expression of the thyroglobulin gene by testing in a transient expression transfection assay various constructs from the 5' sequences of a cloned at thyroglobulin gene in differentiated follicle cells (FRTL-5) and in mutant derivatives of these and other thyroid-derived cell strains. III. We are attempting to culture the elements of and to reconstruct in vitro functional olfactory epithelium from rats. They sensory neurons of the olfactory epithelium are renewed throughout life from a stem cell population. Development of this system would make available the first mammalian neuroblast to neuron cell culture system. It is hoped that basic issues in olfactory sensory physiology can be explored with this system. IV. The germinal cells of the testis in the F1 generation from a BALB/c x Czech mouse cross are arrested at prophase I of meiosis. Such arrested cells can be isolated in quantity and divide in culture and as heterokaryons with dividing somatic cells.